This invention relates to air-cooled condensing systems and methods and more particularly to a system that is thermodynamically more efficient and simpler in physical execution than current state of the art air-cooled condensing systems.
Numerous condensing process arrangements have been introduced into the air-cooled condenser (ACC) industry since their introduction in the 1930's. Most did not survive and over time one system gained predominance in the industry. This system employed a single pressure, series flow, two-stage condensing process. The first stage was arranged for parallel flow of steam and forming condensate and was referred to as a condensing (or K) section. The second stage was arranged for counter flow of steam and condensate and was referred to as a dephlegmator (or D) section. In this prior condensing system, the entire condensing process takes place at a nearly constant, or single, pressure. These systems are commonly referred to in industry as K-D type. Many hundreds have been installed worldwide in all extremes of climatic conditions demonstrating reliability over many decades of operation.
The main reason for the adoption of the K-D system as the industry standard was because it offered reliable performance over a wide range of climatic extremes along with reasonably efficient condensing performance when employed in conjunction with multi-row fin tube heat exchangers, the only type available at the time. Cooling air entering a multi-row fin tube heat exchanger steadily increases in temperature as it traverses in the cross-flow direction from the first to the last fin tube row resulting in a decrease in row-to-row condensing rates. This causes premature completion of condensation in the first tube rows of the heat exchanger. As a consequence portions of the first rows of tubes fill with non-condensibles, commonly referred to as “dead zones”, with a resultant total loss of heat exchange where this condition is present. Furthermore, the presence of dead zones presents a strong potential for freeze-up and damage to the tubes during cold weather operation. Such events can result in severe economic consequences. To combat this problem and achieve more uniform condensing rates in multi-row exchangers, designers incorporated variable fin spacings on the tubes with the fin pitch set steadily tighter from the first to the last row. This however only partially mitigated the presence of “dead zone” and it also reduced the amount of fin surface that could be deployed because the fins in the first rows could be only loosely pitched.
The two-stage K-D condensing process referred to above was devised in order to overcome the problems of dead zones in multi-row fin tube heat exchangers. In this process steam first enters the K section heat exchangers from above. By limiting the length of the K tubes and by properly modulating airflow, condensation is not allowed to complete in this section and some steam exits all tube rows at the bottom under all operating conditions. However, the conventional K-D condensing process has other problems. Condensate draining from the K section flows parallel to the downward flowing steam and therefore has a very short residence time in the K tubes. Because it flows in the bottom of the tubes, it is in contact with the coldest metallic portions of the tubes. This results in some sub-cooling of the condensate. The condensate is then routed to the condensate tank in a system of drainpipes that are exposed to cold air. This causes further sub-cooling of the condensate. Sub-cooling of condensate is deleterious because it decreases thermodynamic efficiency and, more importantly, increases the dissolved oxygen content of the condensate. Dissolved oxygen in the condensate creates serious corrosion problems in the overall steam cycle. Separate condensate deaerators are frequently incorporated to control the amount of sub-cooling occurring in K-D condensing systems, adding to the complexity and cost of the system.
Steam leaving the K section is collected in a header and then introduced from below into the second stage D section. The size of the D section can vary between as little as 8% to as much as 25% of the overall deployed condenser heat transfer surface. Condensation finally completes near the very top of the D section with the remaining interior tube volume being filled with non-condensibles. These are continuously removed by ejection equipment. All condensate formed in the D section drains downward in direct contact with and counter to the direction to the up-flowing steam. This arrangement results in a reliable highly freeze-proof condensing system. Subcooling of condensate in the D section is much less than in the K section because of increased residence time and increased contact from turbulence with up flowing steam. Although the K-D system meets the crucial requirement of minimizing unwanted “dead zones” in the condenser and providing reliable operation in extreme cold weather conditions, inherently high internal steam side pressure drops degrade its performance. These result from the fact that the steam must pass in series through two stages of fin tubes plus a steam transfer header, producing considerable friction losses plus additional turning and acceleration losses leaving and entering the two sets of fin tubes. These parasitic pressure losses produce a corresponding drop in the saturation temperature of the steam, which reduce the temperature difference potential between steam and cooling air, and thus the efficiency of the heat exchangers.
The steam path between the turbine and start of condensation in the K sections is frequently torturous and long. Typically the associated steam ducting involves four 90-degree turns, lengthy laterals, risers and upper distribution ducts before the steam enters the fin tubes. This is both costly and again depresses the saturation temperature of the steam due to the accompanying pressure drops, thereby degrading heat exchanger performance for the same reasons as noted above. The only way to compensate for these parasitic losses up to now has been to increase the physical size of the ACC.
In addition to the requirement for the above noted condensate deaerator, condensate drain lines and steam transfer header, several additional features must typically be incorporated in K-D systems for proper operation. These additional features include a pressure equalizing line between turbine exit and the condensate tank, a drain pot plus transfer pumps and piping to continuously drain condensate out of the main steam duct, a condensate tank to collect the condensate draining from the transfer headers, and condensate drain piping insulation and heat tracing to prevent freezing during cold weather operation.
In the last fifteen years much larger single row fin tubes have become commercially available and are now the industry standard because of their improved economics. The advent of the single row fin tube bundle represented a milestone in the evolution of ACC's in that the problem of variable-condensing rates in multiple tube rows is eliminated. It also permits the deployment of the densest possible fin pitch resulting in maximum deployment of heat exchange surface per unit of exchanger face area.